To determine the refractive-index profile in optical fibres the so-called near-field technique is generally used; this technique gives fairly accurate results without requiring any particular fiber preparation or sophisticated measurement apparatus.
According to this technique, a fiber end is illuminated and power distribution is examined at the opposite end by a suitable detector. According to whether lost power or guided power are measured there are two types of near-field techniques known as the "refracted nearfield" or the "bound near field" technique respectively.
The refracted near-field technique has been suggested by W. J. Stewart in the paper "A new technique for measuring the refractive index profile of graded optical fibres", presented at the 1977 International Conference on Integrated Optics and Optical Fibre Communication" (IOOC'77), Tokyo, 18-20 July 1977, paper C 2--2, pages 395-398.
The disadvantages of this method are that it does not exploit fiber propagation characteristics and the measurement is not carried out at the wavelengths used for the transmission once the fiber has been installed; as to the latter point, it is worth noting that the refractive index varies with the wavelength (refractive-index profile dispersion) and this dispersion is seldom accurately known, so that it can be difficult to obtain the profile at the operational wavelength.
An example of the bound near-field technique is described by G. Coppa, P. Di Vita and U. Rossi in the paper "A simple technique for the measurement of the refractive-index profile of monomode fibres" presented at the Fourth International Conference on Integrated Optics and Optical Fibre Communication, Tokyo, June 27-30, 1983, paper 28 S A2--2, pages 38-39.
The method described is based on the fact that the near-field intensity transmitted by a monomode fiber is proportional to the square of a transverse electromagnetic field component E satisfying the wave equation: EQU .DELTA.E+[k.sup.2 n.sup.2 (r)-.beta..sup.2 ].multidot.E=0 (1)
where k=2.pi./.lambda.=wave number in vacuum; n(r)=refractive index at distance r from the fiber axis; .beta.=longitudinal mode propagation constant, and .DELTA.=Laplacian operator. The value of n(r) can be extracted by inverting the said wave equation.
This method can give inaccurate results since it requires complex mathematical calculations (for example, the digital calculation of a second derivative) which can cause errors and must be carried out on a measured quantity, which in turn can be error-affected.